The addition of additives to molten polymers has been accomplished by several means. One such means is blending the additives and the polymer chips together in the polymer chip dryer or in the storage hopper prior to extruding the polymer chips and the additives into strands for pelletizing. Another method for introducing additives into a polymer melt stream is to inject the additives at the throat, the mixing zones, or the vent of the extruder and to allow the extrusion process to fully blend the additives into the polymer components. A third method of introducing additives into a polymer melt stream involves injecting the additives into static mixing elements located downstream of the extruder to fully blend the additives into the polymer components.
Problems arise, however, in that some additives may be heat sensitive and may also cause polymer degradation or other undesirable reactions with the polymer if blended with the polymer before extrusion into polymer strands. Moreover, some additives such as, for example, zinc stearate, can cause extruder screw slippage.
A way to overcome such problems is to introduce the additives into a polymer melt stream after extrusion of the polymer into strands for pelletizing. One such method is to coat the polymer pellets with the additives after the polymer extrusion process has occurred. A problem arises, however, in that for additives that amount to less than about 1 percent of the concentration of the total polymer product, this method does not generally result in a good uniform dosing of additive to polymer.
A need, therefore, exists for a method of introducing additives into the polymer that overcomes the above-discussed limitations.